1. Field of the Invention
The present invention relates generally to wireless communications, and more particularly, to transferring data in a carrier aggregation environment.
2. Description of the Related Art
Wireless communication systems have been widely deployed to provide various communication services such as voice, video, packet data, broadcast, and messaging. These systems may be multiple-access systems capable of supporting communication for multiple users by sharing available system resources.
Typically, a wireless communication medium is subject to impairments due to path loss, signal attenuation, multi-path fading, and shadowing, for example. Thus, a receiver (e.g., user equipment, or Evolved Node B (eNB)) may find data packets lost over a radio channel or receive erroneous data packets that are discarded at the receiver. In this case, a Radio Link Control (RLC) entity at the receiver may discover that some sequence numbered data packets are missing. Accordingly, the RLC entity at the receiver indicates a positive ACKnowledgement (ACK) for successfully received data packets and Negative ACKnowledgment (NACK) for lost data packets to an RLC entity at a transmitter. Based on the status report, the transmitter RLC entity may re-transmit the NACK data packets and flush the ACK data packets.
In high data rate wireless communication systems, a data channel is shared among multiple User Equipments (UEs). The wireless network performs a scheduling operation and provides each of the UEs with respective grant such that each of the UEs utilizes transmission resources indicated in the grant to transmit data packets. A given UE may have many data services or applications, such as Internet browsing, Wireless Application Protocol (WAP), email client, and messenger, running on the UE at any given instant. Each of these data services can be mapped to a single radio bearer. There are few radio bearers supporting only signaling information exchange between the UE and the network, as a plurality of radio bearers is active in the UE. Typically, the UE distributes the transmission grant provided by the wireless network among one or more active radio bearers. While scheduling, the UE considers priority and data rate requirements of the radio bearers to be scheduled. Thus, the UE performs scheduling per radio bearer.
Recently, in wireless communication technologies such as Long Term Evolution (LTE)/LTE Advanced (LTE-A), carrier aggregation has been introduced such that two or more component carriers are aggregated in order to support wider transmission bandwidths up to 100 MHz and for spectrum aggregation. The carrier aggregation technique enables a UE to be configured to aggregate a different number of component carriers originating from the same eNB and of possible different uplink and downlink bandwidths.
In carrier aggregation, the UE may receive transmission grants on all active component carriers in the same sub-frame. Therefore, the UE may have multiple grants available for scheduling the radio bearers. This type of scheduling is also known as logical channel prioritization, and may be serially or jointly executed for all the grants for the concerned component carriers.
Existing techniques fail to handle multiple grants as could occur in carrier aggregation scenarios, leading to inefficient mapping of an RLC status report to any of the grants. As a consequence, smaller status reports may be transmitted to peer entities and incomplete status reports may be generated instead of complete status reports. Moreover, the transmitter RLC entity may not know which of the data packets are transmitted successfully and which are not.
Further, in carrier aggregation systems, the UE triggers a Radio Resource Control (RRC) connection re-establishment procedure when a data packet reaches a maximum number of re-transmissions. The RRC connection re-establishment procedure is an expensive procedure leading to data suspension. As there is no direct mapping between the data packets and component carriers, triggering an RRC connection re-establishment procedure when some of the component carriers have a bad signal condition while the remainder of the component carriers has a good signal condition is undesirable.